This invention relates to a fluid-controlled back-pressure and expansion valve for automatically regulating the pressure of a liquid, especially in a heat-treatment installation, more especially an ultra-high-temperature pasteurizer or sterilizer.
In ultra-high-temperatures pasteurizers or sterilizers, the temperature of the liquid to be treated is rapidly increased, the increased temperature is maintained for a short time in a dwell tube and is then rapidly reduced by expansion in a reduced-pressure chamber. Means for retaining the liquid have to be provided at the outlet of the installation to keep the pressure in the dwell tube at a constant value above the boiling pressure of the liquid and hence to avoid the formation of a two-phase (liquid and vapour) stream which would make it impossible to regulate the dwell time. Since this back-pressure is related to heat-treatment temperature, it has to be above the saturated vapour pressure of the liquid at that temperature. Advantageously, the retaining means may also serve as expansion means for the liquid in the reduced-pressure chamber.
A simple expansion nozzle is normally used for this dual function. With an expansion nozzle, the back-pressure is proportional to the square of the throughflow. In other words, an expansion nozzle is particularly sensitive to variations in throughflow and also to the temperature, to the temperature gradient resulting from expansion and to the viscosity of the liquid to be treated. The soiling of the nozzle itself after long production cycles accentuates these adverse effects. During the cleaning cycles, it is desirable that the throughput of cleaning preparation be higher than the normal production throughput. In fact, in accordance with its design features, an expansion nozzle of the type in question is only suitable for a very narrow throughput and temperature range and for the particular type of liquid to be treated. Efforts have been made to improve the performance of the nozzles by lengthening them and increasing the nozzle diameter so that expansion actually begins within the nozzle itself. With a nozzle of this type, the back pressure increases linearly with the throughput so that the nozzle may be used over a wider range of throughputs. However, neither one nor the other is suitable in the starting phase of continuous sterilizers comprising long dwell times of the order of 2 to 15 minutes. During the temperature increase period of the dwell tube, which is filled with water without expansion occurring, the nozzles allow a very high throughflow of liquid, the counter-pressure for which they provide being inadequate. A two-phase stream is thus formed, the expansion chamber becomes filled with cold water and the temperature-control and level-control systems no longer function correctly.
Another type of back-pressure means consists of a membrane pressure controller which controls a valve through a valve pin resting on the controlled side of the membrane and a screw which regulates the tenion of a spring resting on the controlling side of the membrane. The spring may optionally be replaced by a fluid under pressure on the controlling side, as in the case for example in U.S. Pat. No. 2,335,250. The spring-loaded valves require frequent adjustment. The valve pins tend to become blocked in high-temperature operation. The passage between the valve and its seat is narrow and is easily fouled. In the equilibrium state, the pressures above and below the membrane are identical. The valve responds to variations in pressure through an elastic membrane of relatively large surface area which is thus fragile. In addition, there is no removal opening for the liquid ensuring expansion.
A last type of known valve for adjustment of the pressure of a liquid is provided with a large-surface diaphragm carrying a double membrane mounted in an annular ring and reinforced by metallic sectors. The diaphragm is traversed by a rod hollowed out at its end to form outlet orifices for the liquid. The pressure on the membrane on the controlling side is provided by compressed air. In the operational state, the pressures are equal on both sides of the membrane. A valve of the type in question is suitable for temperatures of up to about 120.degree. C. Beyond this limit, it becomes unstable and tends to "pump". The diaphragm is deformed and the valve soon becomes unuseable. In addition, it is not designed to be connected to a vacuum. Finally, the types of membrane valve described above do not comply with the hygiene-related safety requirements for aseptic operation. Their construction, particularly in the vicinity of the zones where the membrane is fixed, makes it difficult for the cleaning preparations to reach certain parts.